Method for scanning for access points in wireless lan system

ABSTRACT

Disclosed is a method for scanning for an access point in a wireless LAN system. An active scanning method comprises the steps of: transmitting a probe request frame to an access point; and receiving, from the access point, a probe response frame corresponding to the probe request frame. The probe response frame may include time information of a beacon transmitted after the probe response frame. Thus, the efficiency of a wireless channel can be improved in a wireless LAN system.

CLAIM FOR PRIORITY

This application claims priority to Korean Patent Application No.2012-0070025 filed on Jun. 28, 2012 and No. 2013-0058167 filed on May23, 2013 in the Korean Intellectual Property Office (KIPO), the entirecontents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

Example embodiments of the present invention relate in general to amethod of scanning an access point and more specifically to a method ofscanning an access point using an active scanning method in a wirelesslocal area network (WLAN) system.

2. Related Art

Various wireless communication techniques are being developed along withthe recent development of an information communication technology. Inparticular, wireless local area network (WLAN) is a technology that canprovide a wireless connection to the Internet in a limited service areasuch as a home or office building using portable terminals, for example,a personal digital assistant (PDA), a laptop computer, and a portablemultimedia player (PMP) based on a wireless frequency technology.

Standards for WLAN technology is developed and standardized by IEEE802.11 working group (WG) in the Institute of Electrical and ElectronicsEngineering (IEEE) 802.11. IEEE 802.11a provides a transfer rate of 54Mbps using 5 GHz unlicensed band. IEEE 802.11b provides a transfer rateof 11 Mbps by using direct sequence spread spectrum (DSSS) in 2.4 GHzband. IEEE 802.11g provides a transfer rate of 54 Mbps by usingorthogonal frequency division multiplexing (OFDM) in 2.4 GHz band. IEEE802.11n provides a transfer rate of 300 Mbps for two spatial streams byusing multiple-input multiple-output OFDM (MIMO-OFDM). IEEE 802.11nsupports a channel bandwidth of up to 40 MHz and, in this case, providesa transfer rate of 600 Mbps.

Along with the wide deployment of WLAN and the diversification ofapplications using WLAN, new WLAN techniques are increasingly needed tosupport higher throughput than IEEE 802.11n. Very high throughput (VHT)WLAN is one of IEEE 802.11 WLAN techniques, which is proposed to supporta data processing rate of 1 Gbps or more. In particular, IEEE 802.11acis developed as a standard for providing a very high throughput on the 5GHz band, and IEEE 802.11ad is developed as a standard for providing avery high throughput on the 60 GHz band.

In a system based on the WLAN techniques, a method of scanning an accesspoint includes an active scanning method and a passive scanning method.

In the active scanning method for the access point, a terminal (station)transmits a probe request frame to an access point, and upon receivingthe probe request frame, the access point responds by transmitting aprobe response frame to the terminal. Generally, the probe responseframe includes much information and thus occupies a wireless channel fora long time.

In an environment in which there are a plurality of access points and aplurality of terminals, the respective probe request frames of theplurality of terminals cause a considerable number of probe responseframes, which competitively occupy a wireless channels. Thus, terminalsmay not receive a probe response frame from a desired access point in ashort time and should stay awake while receiving a probe response framefrom an even undesired access point in order to receive the desiredprobe response frame.

In addition, since the terminal does not know a channel in which thedesired access point is present, the terminal should sequentiallyperform the access point scan process for each channel. In this case,the terminal moves to another channel after sufficiently waiting untilthe terminal receives the probe response frame of the desired accesspoint.

That is, in the active scanning method for the access point, it takes along time to receive the probe response frame of the desired accesspoint.

SUMMARY

Accordingly, example embodiments of the present invention are providedto substantially obviate one or more problems due to limitations anddisadvantages of the related art.

Example embodiments of the present invention provide an active scanningmethod that is performed by a terminal using a probe response frameincluding only information needed to scan an access point.

Example embodiments of the present invention also provide an activescanning method that is performed by an access point using a proberesponse frame including only information needed to scan the accesspoint.

In some example embodiments, a method of performing an active scan by aterminal includes transmitting a probe request frame to an access point,and receiving a probe response frame corresponding to the probe requestframe from the access point, in which the probe response frame includestime information of a beacon transmitted from the access point after theprobe response frame.

The method may further include receiving the beacon from the accesspoint at a time indicated by the time information of the beacon.

The terminal may operate in a power save mode until the time indicatedby the time information of the beacon after receiving the probe responseframe.

The terminal may scan an access point on another channel until the timeindicated by the time information of the beacon after receiving theprobe response frame.

The time information may include information on an interval before thebeacon is transmitted.

The time information may include information on a time at which thebeacon is transmitted.

The time information may include time information of a next full beaconor time information of a next short beacon.

The probe response frame may be a short probe response frame.

In other example embodiments, a method of responding to an active scanby an access point includes receiving a probe request frame from aterminal; generating a probe response frame in response to the proberequest frame; and transmitting the probe response frame to theterminal, in which the probe response frame includes time information ofa beacon transmitted after the probe response frame.

The method may further include transmitting the beacon at a timeindicated by the time information of the beacon.

The time information may include information on an interval before thebeacon is transmitted.

The time information may include information on a time at which thebeacon is transmitted.

The time information may include time information of a next full beaconor time information of a next short beacon.

The probe response frame may be a short probe response frame.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments of the present invention will become more apparentby describing in detail example embodiments of the present inventionwith reference to the accompanying drawings, in which:

FIG. 1 is a conceptual view showing a configuration of an IEEE 802.11WLAN system according to an embodiment of the present invention;

FIG. 2 is a conceptual view showing an association process for aterminal in an infrastructure BSS;

FIG. 3 is a conceptual view showing a data transmission process of anaccess point according to an embodiment of the present invention;

FIG. 4 is a conceptual view showing an active scanning method inmultiple channels;

FIG. 5 is a conceptual view showing a passive scanning method inmultiple channels;

FIG. 6 is a flowchart showing an active scanning method according to anembodiment of the present invention; and

FIG. 7 is a conceptual view showing an active scanning method accordingto an embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Since the present invention may be variously modified and have severalexemplary embodiments, specific exemplary embodiments will be shown inthe accompanying drawings and be described in detail in a detaileddescription.

However, it should be understood that the particular embodiments are notintended to limit the present disclosure to specific forms, but ratherthe present disclosure is meant to cover all modification, similarities,and alternatives which are included in the spirit and scope of thepresent disclosure.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first component may be designatedas a second component, and similarly, the second component may bedesignated as the second component. The use of the term of ‘and/or’means that combination of a plurality of related and described items orone items among a plurality of related and described items is included.

When it is mentioned that a certain component is “coupled with” or“connected with” another component, it may be understood that anothercomponent can exist between the two components although the componentcan be directly coupled or connected with the another component.Meanwhile, when it is mentioned that a certain component is “directlycoupled with” or “directly connected with” another component, it has tobe understood that another component does not exist between the twocomponents.

In the following description, the technical terms are used only forexplaining a to specific exemplary embodiment while not limiting thepresent disclosure. Singular forms used herein are intended to includeplural forms unless explicitly indicated otherwise. It will be furtherunderstood that the terms “comprises,” “comprising,” “includes,” and/or“including” when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or a combinationthereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. Terms suchas terms that are generally used and have been in dictionaries should beconstrued as having meanings matched with contextual meanings in theart. In this description, unless defined clearly, terms are not ideally,excessively construed as formal meanings.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Indescribing the invention, in order to facilitate the entireunderstanding of the invention, like numbers refer to like elementsthroughout the description of the figures and the repetitive descriptionthereof will be omitted.

Throughout this specification, a station (hereinafter also referred toas an STA) is any functional medium including a medium access control(MAC) and wireless-medium physical layer (PHY) interface conforming tothe IEEE 802.11 standard. The STA may include an access point (AP) STAand a non-AP STA. The AP STA may be simply referred to as an accesspoint (hereinafter also referred to as an AP), and the non-AP STA issimply referred to as a terminal.

The STA includes a processor and a transceiver, and may further includea user interface, a display device, and so on. The processor is afunctional unit devised to generate a frame to be transmitted through awireless network or to process a frame received through the wirelessnetwork, and performs various functions to control the STA. Thetransceiver is functionally connected to the processor and is afunctional unit devised to transmit and receive a frame for the STAthrough the wireless network.

The AP may be called a convergence controller, a base station (BS), anode-B, an eNode-B, a base transceiver system, or a site controller, andmay include some or all of functions thereof.

The terminal may be called a wireless transmit/receive unit (WTRU), auser equipment (UE), a user terminal (UT), an access terminal (AT), amobile station (MS), a mobile terminal, a subscriber unit, a subscriberstation (SS), a wireless device, or the like, and may include some orall of functions thereof.

Here, the terminal may include a communication enabled desktop computer,laptop computer, tablet PC, wireless phone, mobile phone, smartphone,e-book reader, portable multimedia player (PMP), portable gamingconsole, navigation device, digital camera, digital multimediabroadcasting (DMB) player, digital audio recorder, digital audio player,digital picture recorder, digital picture player, digital videorecorder, digital video player, etc.

FIG. 1 is a conceptual view showing a configuration of an IEEE 802.11WLAN system according to an embodiment of the present invention.

Referring to FIG. 1, the IEEE 802.11 WLAN system includes at least onebasic service set (BSS). The BBS denotes a set of stations STA 1, STA 2(AP 1), STA 3, STA 4, STA 5 (AP 2) that may be successfully synchronizedto communicate with one another, but does not denote a certain region.

The BBS may be classified into an infrastructure BSS and an independentBSS (IBSS). BBS 1 and BBS 2 denote the infrastructure BSS. BBS 1 mayinclude a terminal STA 1, an access point STA 2 (AP 1) that provides adistribution service, and a distribution system (DS) that connects aplurality of access points STA 2 (AP 1) and STA 5 (AP 2). In BSS 1, theaccess point STA 2 (AP 1) manages the terminal STA 1. BBS 2 may includethe terminals STA 3 and STA 4, the access point STA 5 (AP 2) thatprovides a distribution service, and a distribution system that connectsthe plurality of access points STA 2 (AP 1) and STA 5 (AP 2). In BSS 2,the access point STA 5 (AP 2) manages the terminals STA 3 and STA 4.

The independent BSS (IBSS) is a BSS that operates in an ad-hoc mode.Since the IBSS does not include an access point, there is no centralizedmanagement entity for performing a central management function. That is,in the IBSS, terminals are managed in a distributed manner. The IBSS isa self-contained network, in which all terminals may be mobile terminalsand may be disallowed to access the distribution system (DS).

The access points STA 2 (AP 1) and STA 5 (AP 2) provide a connection tothe DS through a wireless medium for the associated terminals STA 1, STA3 and STA 4. In BSS 1 or BSS 2, generally, a communication between theterminals STA 1, STA 3, and STA 4 is made through the access points STA2 (AP 1) and STA 5 (AP 2). However, when a direct link is established, adirect communication between the terminals STA 1, STA 3, and STA 4 isenabled.

A plurality of infrastructure BBSs may be interconnected through the DS.The plurality of BBSs connected to each other through the DS is calledan extended service set (ESS). STAs included in the ESS may communicatewith each other, and within the same ESS, a terminal may move from oneBSS to another BSS while communicating in a seamless manner.

The DS is a mechanism in which one AP communicates with another AP. Byusing the DS, an AP may transmit a frame to terminals that is associatedwith a BSS managed by the AP, or transmit a frame to a terminal that hasmoved to another BSS. In addition, the AP may transmit and receive aframe to and from an external network such as a wired network. The DS isnot necessarily a network and has no limitation in its form as long as apredetermined distribution service specified in the IEEE 802.11 standardcan be provided. For example, the DS may be a wireless network such as amesh network, or may be a physical structure for interconnecting APs.

An AP scanning method according to an embodiment of the presentinvention, which will be described below, may be applied to the aboveIEEE 802.11 WLAN system and also various networks such as a wirelesspersonal area network (WPAN), a wireless body area network (WBAN), andso on.

FIG. 2 is a conceptual view showing an association process for aterminal in an infrastructure BSS.

In order for a terminal STA to transmit and receive data in aninfrastructure BSS, first, the terminal STA should be associated with anAP.

Referring to FIG. 2, the association process of the terminal STA in theinfrastructure BSS may include: 1) probe step of probing an AP, 2)authentication step of authenticating the probed AP, and 3) associationstep of associating with the authenticated AP.

First, the terminal STA may probe neighboring APs through the probeprocess. The probe process includes a passive scanning method and anactive scanning method. The passive scanning method may be performed byoverhearing beacons that are transmitted by the neighboring APs. On theother hand, the active scanning method may be performed by broadcastinga probe request frame. Upon receiving the probe request frame, the APmay transmit a probe response frame corresponding to the probe requestframe to the terminal STA. The terminal STA may check the presence ofthe neighboring APs by receiving the probe response frame.

Subsequently, the terminal STA performs authentication with the probedAPs, and may perform the authentication with the plurality of APs. Anauthentication algorithm conforming to the IEEE 802.11 standard includesan open system algorithm that exchanges two authentication frames and ashared key algorithm that exchanges four authentication frames. Througha process of exchanging the authentication request frame and theauthentication response frame based on the authentication algorithm, theterminal STA may perform authentication with the AP.

Last, the terminal STA selects one AP from among the authenticatedplurality of APs and performs an association process with the selectedAP. That is, the terminal STA transmits an association request frame tothe selected AP. Upon receiving the association request frame, the APtransmits the terminal STA to an association response framecorresponding to the association request frame. As such, through theprocess of exchanging the association request frame and the associationresponse frame, the terminal STA may perform the association processwith the AP.

FIG. 3 is a conceptual view showing a data transmission process of anaccess point according to an embodiment of the present invention.

Referring to FIG. 3, the AP broadcasts a beacon periodically and maybroadcast a beacon including a delivery traffic indication message(DTIM) at every 3 beacon intervals. The terminals STA 1 are STA2periodically wake up from a power save mode (PSM) and receives thebeacon, and check a traffic indication map (TIM) or DTIM included in thebeacon to determine whether the data to be transmitted to the terminalsis buffered in the AP. In this case, when there is the buffered data,the terminals STA 1 and STA 2 maintain awake and receive the data fromthe AP. When there is no buffered data, the terminals STA 1 and STA 2return to the PSM (that is, a doze state).

That is, when a bit in the TIM corresponding to an AID of the terminalSTA 1 or STA 2 is set as 1, the terminal STA 1 or STA 2 transmits, tothe AP, a power save (PS)-Poll frame (or a trigger frame) that informsthe AP that the terminal STA is awake and ready to receive data. The APmay determine that the terminal STA 1 or STA 2 is ready to receive thedata by receiving the PS-Poll frame and then may transmit the data or anacknowledgement (ACK) to the terminal STA 1 or STA 2. When the APtransmits the ACK to the terminal STA 1 or STA 2, the AP transmits datato the terminal STA 1 or STA 2 at an appropriate time. On the otherhand, when the bit in the TIM corresponding to the AID of the terminalSTA 1 or STA 2 is set as 0, the terminal STA 1 or STA 2 returns to thePSM.

FIG. 4 is a conceptual view showing an active scanning method inmultiple channels.

Referring to FIG. 4, first, the terminal may transmit a probe requestframe to a specific access point or all access points on channel 1 andmay receive a probe response frame that is transmitted from the one ormore access points during a max waiting time. That is, during the maxwaiting time on channel 1, the terminal may receive probe response frame1 from access point 1, receive probe response frame 2 from access point2, and receive probe response frame 3 from access point 3.

After the max waiting time on channel 1, the terminal may move tochannel 2 and transmit a probe request frame to a specific access pointor all access points and may receive a probe response frame that istransmitted from the one or more access points during a max waitingtime. That is, during the max waiting time on channel 2, the terminalmay receive probe response frame 4 from access point 4, receive proberesponse frame 5 from access point 5, and receive probe response frame 6from access point 6.

FIG. 5 is a conceptual view showing a passive scanning method inmultiple channels.

Referring to FIG. 5, first, the terminal may receive beacons transmittedfrom the access point on channel 1 and may move to channel 2 after abeacon interval on channel 1. Subsequently, the terminal may receivebeacons transmitted from the access point on channel 2.

FIG. 6 is a flowchart showing an active scanning method according to anembodiment of the present invention.

Referring to FIG. 6, first, a terminal 10 may transmit a probe requestframe to an access point 20 (S100). Here, the terminal 10 may transmit aprobe request frame to a specific access point or transmit a proberequest frame to unspecific access points (that is, all access points).The terminal 10 may make a request to provide time information of a nextbeacon through the probe request frame. That is, the terminal 10 may setany field that indicates a request to provide the time information ofthe next beacon and may generate a probe request frame including thefield to transmit the probe request frame to an access point.

When the probe request frame is received from the terminal 10, theaccess point 20 may generate a probe response frame including the timeinformation of the next beacon (S110). That is, the access point 20 maytransmit the probe response frame including the time information of thebeacon transmitted after transmission of the probe response frame. Theaccess point 20 may generate a probe response frame including timeinformation of a next full beacon or time information of the next shortbeacon (that is, an auxiliary beacon).

Here, the full beacon denotes a beacon that is generally used in thewireless local area network (WLAN) (IEEE 802.11). The short beacon isused to speed up the passive scan, has a period shorter than the fullbeacon, and denotes a beacon including only essential information forthe scan.

For example, the short beacon may include timestamp information, changesequence information, next full beacon time information, compressedservice set identifier (SSID) information, access network optioninformation, and the like.

The time information of the next beacon may denote an interval beforethe next beacon is transmitted or a time at which the next beacon istransmitted (that is, a target beacon transmission time (TBTT)).

That is, the access point 20 may generate a new probe response frame(that is, a short probe response frame or probe ACK frame) instead of anexisting probe response frame. The new probe response frame may besmaller than the existing probe response frame, and the new proberesponse frame may include the SSID and the next beacon time information(that is, the time information of a beacon transmitted after the proberesponse frame is transmitted) as essential information for the scan.

The access point 20 may transmit the probe response informationincluding the time information of the beacon to the terminal 10 (S120).

Upon receiving the probe response frame from the access point 20, theterminal 10 may acquire the time information (that is, the timeinformation of the beacon transmitted after the probe response frame istransmitted) of the next beacon from the probe response frame (S130).When the time information of the next beacon indicates an intervalbefore the next beacon is transmitted, the terminal 10 may acquire atime at which the next beacon is transmitted based on the current timeand the interval before the next beacon is transmitted.

When the time at which the next beacon is transmitted is acquired, theterminal 10 may return to a sleep state (that is, a power save mode) bythe time at which the next beacon is transmitted or may perform theabove-described active scan in another channel.

The access point 20 may transmit a beacon at a time indicated by thetime information of the next beacon that is included in the proberesponse frame (S140). Here, the access point 20 may transmit a fullbeacon or short beacon. The terminal 10 may receive the beacon at a timeindicated by the time information of the next beacon that is included inthe probe response frame.

FIG. 7 is a conceptual view showing an active scanning method accordingto an embodiment of the present invention.

Referring to FIG. 7, first, the terminal may transmit a probe requestframe on channel 1 and may receive a response thereto during a maxwaiting time. That is, during the max waiting time on channel 1, theterminal may receive probe response frame 1 (that is, probe ACK 1) fromaccess point 1, receive probe response frame 2 (that is, probe ACK 2)from access point 2, and receive probe response frame 3 (that is, probeACK 3) from access point 3.

Here, probe response frames 1 to 3 denote the probe response frameincluding the time information of the next beacon that is shown in FIG.6. That is, since probe response frames 1 to 3 are smaller than theexisting probe response frame, the max waiting time on channel 1 may beset shorter than the existing max waiting time.

Upon receiving the probe response frame transmitted during the maxwaiting time on channel 1, the terminal may determine an associationtarget access point based on signal strength of the probe responseframe. That is, the terminal may determine, as the association targetaccess point, an access point that transmits a probe response framehaving highest signal strength among probe response frames 1 to 3 or maydetermine, as the association target access point, an access point thattransmits a probe response frame satisfying a predetermined referenceamong probe response frames 1 to 3.

For example, when probe response frame 1 has the highest signalstrength, the terminal may determine access point 1 as the associationtarget access point and may acquire a transmission time of next beacon 1or next auxiliary beacon 1 (that is, short beacon 1) based on the timeinformation of the next beacon included in probe response frame 1. Here,the terminal may determine to receive a beacon at a transmission time ofnext beacon 1.

When plenty of time remains before the transmission time of next beacon1 (that is, a time for scanning an access point in another channel), theterminal may scan an access point in another channel. That is, theterminal may move to channel 2 and transmit the probe request frame.During the max waiting time on channel 2, the terminal may receive proberesponse frame 4 (that is, probe ACK 4) from access point 4, receiveprobe response frame 5 (that is, probe ACK 5) from access point 5, andreceive probe response frame 6 (that is, probe ACK 6) from access point6.

Here, probe response frames 4 to 6 denote the probe response frameincluding the time information of the next beacon that is shown in FIG.6. That is, since probe response frames 4 to 6 are smaller than theexisting probe response frame, the max waiting time on channel 2 may beset shorter than the existing max waiting time.

Upon receiving the probe response frame transmitted during the maxwaiting time on channel 2, the terminal may determine an associationtarget access point based on signal strength of the probe responseframe. For example, when probe response frame 4 has the to highestsignal strength among probe response frames 4 to 6, the terminal maydetermine access point 6 as the association target access point and mayacquire a transmission time of next beacon 6 or next auxiliary beacon 6based on the time information of the next beacon included in proberesponse frame 6. Here, the terminal may determine to receive a beaconat a transmission time of next beacon 6.

When plenty of time remains before the transmission time of next beacon1 (that is, a time for scanning an access point in another channel), theterminal may scan an access point in another channel. That is, theterminal may move to channel 3 and transmit the probe request frame.During the max waiting time on channel 3, the terminal may receive proberesponse frame 7 (that is, probe ACK 7) from access point 7, receiveprobe response frame 8 (that is, probe ACK 8) from access point 8, andreceive probe response frame 9 (that is, probe ACK 9) from access point9.

Here, probe response frames 7 to 9 denote the probe response frameincluding the time information of the next beacon that is shown in FIG.6. That is, since probe response frames 7 to 9 are smaller than theexisting probe response frame, the max waiting time on channel 3 may beset shorter than the existing max waiting time.

Upon receiving the probe response frame transmitted during the maxwaiting time on channel 3, the terminal may determine an associationtarget access point based on signal strength of the probe responseframe. For example, when probe response frame 9 has the highest signalstrength among probe response frames 7 to 9, the terminal may determineaccess point 9 as the association target access point and may acquire atransmission time of next beacon 9 or next auxiliary beacon 9 based onthe time information of the next beacon included in probe response frame9. Here, when a transmission time of beacon 6 temporarily overlaps atransmission time of beacon 9, the terminal may determine to receive anauxiliary beacon at a transmission time of auxiliary beacon 9 that has alower priority than beacon 9.

When plenty of time does not remain before the transmission time of nextbeacon 1 (that is, a time for scanning an access point on anotherchannel), the terminal may move to channel 1 to receive beacon 1 at atransmission time of beacon 1, move to channel 2 to receive beacon 6 ata transmission time of beacon 6, and then move to channel 3 to receiveauxiliary beacon 9 at a transmission time of auxiliary beacon 9.

According to an embodiment of the present invention, a time during whicha wireless channel is occupied by the probe response frame may bereduced by using a probe response frame (that is, a short probe responseframe or probe ACK) including information needed to scan an accesspoint, thus enhancing efficiency of a wireless channel.

In addition, the terminal may be aware of a transmission time of abeacon through time information of a next beacon included in the proberesponse frame and scan an access point on another channel until thenext beacon is transmitted based on the transmission time, thus reducingtime consumed in scanning the access point.

While the example embodiments of the present invention and theiradvantages have been described in detail, it should be understood thatvarious changes, substitutions and alterations may be made hereinwithout departing from the scope of the invention.

What is claimed is:
 1. A method of performing an active scan by aterminal, the method comprising: transmitting a probe request frame toan access point; and receiving a probe response frame corresponding tothe probe request frame from the access point, wherein the proberesponse frame includes time information of a beacon transmitted fromthe access point after the probe response frame.
 2. The method of claim1, further comprising receiving the beacon from the access point at atime indicated by the time information of the beacon.
 3. The method ofclaim 2, wherein the terminal operates in a power save mode until thetime indicated by the time information of the beacon after receiving theprobe response frame.
 4. The method of claim 2, wherein the terminalscans an access point on another channel until the time indicated by thetime information of the beacon after receiving the probe response frame.5. The method of claim 1, wherein the time information includesinformation on an interval before the beacon is transmitted.
 6. Themethod of claim 1, wherein the time information includes information ona time at which the beacon is transmitted.
 7. The method of claim 1,wherein the time information includes time information of a next fullbeacon or time information of a next short beacon.
 8. The method ofclaim 1, wherein the probe response frame is a short probe responseframe.
 9. A method of responding to an active scan by an access point,the method comprising: receiving a probe request frame from a terminal;generating a probe response frame in response to the probe requestframe; and transmitting the probe response frame to the terminal,wherein the probe response frame includes time information of a beacontransmitted after the probe response frame.
 10. The method of claim 9,further comprising transmitting the beacon at a time indicated by thetime information of the beacon.
 11. The method of claim 9, wherein thetime information includes information on an interval before the beaconis transmitted.
 12. The method of claim 9, wherein the time informationincludes information on a time at which the beacon is transmitted. 13.The method of claim 9, wherein the time information includes timeinformation of a next full beacon or time information of a next shortbeacon.
 14. The method of claim 9, wherein the probe response frame is ashort probe response frame.